I like the packs idea personally. i don't think it fragments the player pool, since they're all still playing on the same ladder and each player is no more or less likely to get matched against another player than they are at the moment.
It's also nice in that you can perhaps have a lower level pack in addition to a members-only pack, so that lower level players can join the ladder (perhaps not too low to discourage alts) as well as enable member only features like bomb and commanders on templates that have those and merit inclusion.
My criticism to packs is I find it harder to see how "fair" the ratings are. For example, say Player A is in 1st place, with a rating of 1800. This player only plays on the base packs, and does not bother to even play on the others. now player B is 1780. Player B plays on every single pack. Is player A really considered the "best player"? I am not so sure.
MotD said that you'd get some bonus points every time you play a game on a non-base-pack template so i don't think that would be an issue. If you want to get 1st, you would probably need to use all packs, but the huge number of templates needn't turn off the average player who just wants good strategic games.
If these low level players join the ladder they start with 1500 elo points and play against players, which are way better than them. This can be really frustrating.
On other WL ladders you start from the bottom, not the middle, which really makes more sense for a "ladder". It also helps discourage runs, since you need to play more games to get the games against top players quickly. I think this would be a good feature for the majority of players, though maybe less ideal for the elite. However, the problem isn't really a lack of elite players - more a lack of average strategic players.